1. Field of the Invention
The present invention generally relates to crystal growth and, in particular, to the growth of single crystals of ternary chalcopyrite compounds of the general formula II-IV-V.sub.2, and I-III-VI.sub.2. According to this rotation, the Roman numerals I through VI represent elements from columns IB through VIB of the Periodic Table, where I is Cu, Ag, Au or mixtures thereof; II is Zn, Cd, Hg or mixtures thereof; III is Al, Ga, In, Tl or mixtures thereof; IV is Si, Ge, Sn or mixtures thereof; V is P, As, Sb or mixtures thereof; and VI is S, Se, Te, or mixtures thereof.
2. Statement of the Prior Art
Single crystals of elemental group II-IV-V.sub.2 compounds such as zinc germanium phosphide, as well as single crystals of elemental group I-III-VI.sub.2 compounds, such as silver gallium selenide, can be used for shifting the wavelength of various laser sources into the mid-infrared region of the spectrum (approximately 2 to 8 microns). One of two non-linear optical processes for wavelength shifting is Second Harmonic Generation (SHG), which is used to double the frequency (halve the wavelength) of far-infrared carbon dioxide lasers. The other optical process is Optical Parametric Oscillation (OPO) which can be used to double the wavelength of various near-infrared solid-state lasers. In either case, the efficiency at which the wavelength of the laser radiation can be shifted into the mid-infrared can generally be improved by increasing the length of the crystal as well as by improving the transparency and overall optical quality thereof. In addition, the frequency conversion process is possible only along certain crystallographic directions known as the "phase matching" directions in the material. Since the highest possible conversion efficiency is required for the laser applications in which these crystals are employed, it is desirable to produce high optical quality crystals with low absorption losses in the direction required for phase-matching.
The most common problems that occur during attempts at growing large crystals are cracking and the growth of multiple crystals. In addition, deficiencies in the final composition of the crystals often occur which lead to decreased transparency due to absorption and/or scattering losses. One final difficulty relating to the growth of phosphorus-containing compounds such as ZnGeP.sub.2 is the danger of explosion when attempting to synthesize the compound from the individual elements.
B. Ray et al, Phys. Star. Sol. 35,197 (1969), describes two methods for synthesizing ZnGeP.sub.2, one involving direct reaction of the starting elements and one two-step approach in which Ge is combined with pre-reacted ZnP.sub.2. The heating cycle for the direct synthesis process features 48 hour and 16 hour soaks at 530.degree. C. and 900.degree. C. respectively to avoid explosion of the silica ampoule. Their subsequent crystal growth attempts, however, yielded only cracked, polycrystalline ingots with poor transparency.
One slightly more successful approach was published in 1973 (J. Elect. Mat. 2,445) by Buehler and Wernick. Stoichiometric amounts of zinc germanium and phosphorus, as well as excess phosphorous or zinc diphosphide, for the vapor phase, were loaded into a vitreous carbon boat and sealed into an evacuated quartz ampoule. The ampoule was heated stepwise in a resistance wound, single zone furnace at an average rate of 125.degree. C./day to above the melting temperature. The furnace was then cooled at rates approaching 25.degree. C./day through the freezing point and subsequent solid state phase transition and thereafter at a rate of 50.degree.-75.degree. C./day to room temperature. Temperature gradients during cooling varied from 0.24.degree. C./cm. to 2.0.degree. C./cm. For temperature gradients of less than 0.4.degree. C./cm, the process resulted in crack-free, single crystals weighing between 7 and 10 grams. Higher temperature gradients resulted in large grain polycrystalline ingots with numerous cracks and bubbles. In either case, the crystals were randomly oriented and too small to yield samples for device applications. Unfortunately, these and other previous attempts have failed to provide larger, oriented single, crack-free crystals of this compound.
R. S. Feigelson and R. K. Route, Opt. Eng. 26, 113, 1987, describe the growth of large crack and twin-free single crystals of two other chalcopyrite compounds, AgGaS.sub.2 and AgGaSe.sub.2, with diameters from 28 mm to 37 mm and lengths up to 100 mm by the standard Bridgman-Stockbarger method. The essential features of their process were 1) the use of tapered vertical ampoules, and 2) the use of seed crystals oriented along the [001] c-axis. These features were necessary to successfully grow crack-free crystals using a vertical geometry, because both of these materials expand along their c-axis during cooling. Consequently, these materials cannot be grown by this technique along the orientations required for most laser applications, since these orientations are far from the c-axis.
Chalcopyrite crystals typically require a post-growth heat treatment in order to improve their transparency for laser applications. In the case of the I-III-VI.sub.2 materials AgGaSe.sub.2 and AgGaS.sub.2, the poor as-grown transparency is due to light scattering by second-phase Ga.sub.2 Se.sub.3 and Ga.sub.2 S.sub.3 precipitates respectively. Feigelson and Route teach a method for annealing these crystals in sealed quartz ampoules in the presence of excess Ag.sub.2 Se and Ag.sub.2 S respectively which produces near-theoretical transparency by dissolving the precipitates.
II-IV-V.sub.2 crystals like ZnGeP.sub.2 are generally free of scattering centers, but their transparency is often limited by a defect-related absorption band which extends from the band edge into the infrared portion of the spectrum. Y. V. Rud and R. V. Masagutova (Sov. Tech. Phys. Lett. 7,72, 1981) describe an annealing technique for reducing this absorption in the edge region which involves annealing thin ZnGeP.sub.2 samples in sealed quartz ampoules packed with ZnGeP.sub.2 powder for approximately 150 hrs. in the temperature range 450.degree. C. to 550.degree. C. Considerable improvement was observed, but no device-quality crystals were produced.